This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The function of creatine (Cr) and phospocreatine (PCr) in heart, brain and skeletal muscles are well studied by different techniques and also by the routine T1 and T2 MRI techniques. In human, 90% of the total creatine is found in skeletal muscles. Various studies show that during physical exercise, creatine is being transformed to phospocreatine in human calf muscle. The variation of creatine and phospocreatine in human calf muscle especially in different parts viz., Tibialis Anterior (TA), Medial Gastrocnemius (MG) and Lateral gastrocnemius (LG) are different depending on the type of exercise. The aim of the project is to determine the distribution of calf muscle activity during physical exercise using CEST MRI with quantifying activity across multiple axial sections and to determine if there are differences among portions of active muscle. Pre-and post-exercise CEST measurements at 1.8 ppm (Creatine;Cr) and 2.5 ppm (Phospocreatine;PCr) were assessed to see change in CEST contrast in the three muscles (portions). The proposed CEST imaging offers better image contrast than the 31P MRI. The purpose of this study is to exploit inherent contrast with CEST and correlate with 31P data of the literature. These results will enable us to determine specificity of CEST in to discriminate creatine and phosphocreatine variations in calf muscle. With these studies, we hope to gain additional insight into the spatial distribution Cr and PCr in calf muscle. Future goal is to get co-registered CEST and 31P in-vivo images of Cr and PCr to evaluate the two techniques for increased reliability.